1. Field of the Invention
The present invention relates to a method for measuring the amount of acidic metal carbonyl hydrides and derivatives thereof in a liquid. In one aspect, it pertains to the relationship between the transition metal carbonyl hydride concentration in an organic stream and the conductivity and/or the pH of the stream. In another aspect, it pertains to a method of controlling the air demetallating of a crude product stream by measurement of either the conductivity or pH of the demetallated stream.
2. Description of the Background
Catalytic processes involving acidic transition metal carbonyl hydrides and derivatives thereof are well known. For example, acidic metal carbonyl hydrides are active catalysts for the homologation of alcohols, the carbonylation of cyclic ethers, dialkyl acetals, ortho esters, and esters, and the hydrogenation, hydrosilation, hydrocarboxylation, and hydroformylation of alkenes. Typical metal carbonyl hydrides employed for such reactions include those of the metals of Groups VI, VII, and VIII. For example, the hydroformylation or oxo process is used for the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (synthesis gas) with carbon compounds containing olefinic linkages in the presence of a carbonylation catalyst. Reaction conditions typically comprise synthesis gas pressures of from about 1500 to 4500 psig and temperatures of from about 150.degree. to 450.degree. F.
The oxo process is a particularly attractive method for the preparation of valuable aldehydes. A great variety of olefins may be used in the process. Thus, straight- and branched-chain olefins and diolefins, olefinic fractions from the hydrocarbon synthesis process, thermal or catalytic cracking operations, and other sources of hydrocarbon fractions containing olefins may be used as starting materials. Not only olefins, but most organic compounds possessing at least one non-aromatic carbon-carbon double bond may also be reacted by this method. Typically, the catalyst may be added in the form of a cobalt compound such as cobalt carbonyl, a cobalt soap, or a phosphine coordination complex of a cobalt compound. Under the conditions of the reaction, the catalyst is present as an acidic cobalt carbonyl hydride, e.g., HCo(CO).sub.4 or HCo(CO).sub.3 PR.sub.3. Catalysts which do not contain alkyl or aryl phosphine ligands are generally referred to as unmodified catalysts. The oxo process is discussed in great detail in U.S. Pat. Nos. 3,518,319 and 3,239,569, which are incorporated by reference herein.
Efficient recycling of the catalysts often requires that the catalyst be removed from the crude product before purification of the product. For example, distillation of a crude product before removal of an acidic metal carbonyl hydride may result in loss of catalyst due to thermal decomposition.
In particular, the unmodified cobalt carbonyl catalyst has been widely used for synthesis of aldehydes by the oxo process using what is generally referred to as an overflow reactor design. The crude aldehyde, reaction solvent, and cobalt carbonyl hydride as effluent from the oxo reactor are sent to the crude aldehyde refining distillation columns. It is well known in the art that the cobalt carbonyl hydride contained in the crude aldehyde product stream must be removed prior to aldehyde rectification. Thermal treatment of the cobalt carbonyl hydride in the base of the aldehyde rectification column results in the decomposition of the cobalt carbonyl hydride to metal plate, and to some degree a small quantity of cobalt carbonyl hydride will volatilize into the overhead product. In either case the addition of cobalt carbonyl hydride to the base of an aldehyde rectification column is undesirable.
A number of methods have been devised for the recovery and recycling of cobalt catalysts from organic streams. U.S. Pat. No. 3,369,050 discloses the process of distilling the product from the catalyst in the presence of a controlled concentration of a protecting agent such as a mixture of propylene and carbon monoxide, and thereby protecting the catalyst from decomposition. U.S. Pat. No. 3,539,634 describes the removal of tarry constituents from the liquid residue after distillation by passing the residue over a solid adsorbent which is effective in selective adsorption of the tarry constituents without significant adsorption of the catalyst. The use of a cobalt catalyst which is supported on alumina and is thus easily separable from the product stream is disclosed in U.S. Pat. No. 3,991,119. U.S. Pat. No. 4,060,557 describes the use of a wiped film evaporator for separating the products from the catalysts. The evaporator has rotating wiper blades which mechanically produce a thin film and continuously wipe this film on a heated surface. This mechanical wiping action provides a more rapid removal of the catalyst residue or bottoms. The process thus allows for minimal build-up of catalyst residue, reduced catalyst decomposition, and rapid continuous processing.
A preferred process for catalyst recovery from the product stream involves treating the product stream with oxygen to convert the cobalt carbonyl hydride to a water soluble cobalt salt. The cobalt salt may then be removed from the product stream by contact with water. This process is known as air decobalting. U.S. Pat. No. 4,225,458 teaches a method for the regeneration of the catalyst from the cobalt salt which comprises contacting the water solution of the cobalt salt with a soap derived from the heavy oxygenated bottoms fraction from the distillation unit.
Of the above-mentioned methods for the recovery of recycle of the cobalt catalyst, the air decobalting method is preferred. However, the quantity of air used for the decobalting must be controlled. As disclosed in U.S. Pat. No. 3,409,648, addition of excess air to the product stream results in oxidation of desired aldehyde product to undesired carboxylic acid. Thus, the quantity of air added to the product stream must be limited to the minimum amount necessary to convert all of the cobalt carbonyl hydride to cobalt salts. Addition of too little air will result in incomplete conversion of cobalt carbonyl hydride to cobalt salts and loss of valuable catalyst through decomposition in the distillation step. Addition of too much air will result in oxidation of the desired aldehyde product to undesired carboxylic acid.
Minimizing the quantity of air added in the decobalting step requires a method for measuring the degree of conversion of the cobalt carbonyl hydride to cobalt salts in the product stream. For use in a continuous air decobalting unit, a fast method of measuring the conversion of cobalt carbonyl hydride to cobalt salts which can be used in a feedback system is especially desirable. With such a method of measurement the quantity of air added in the decobalting step could be minimized even when the concentration of cobalt carbonyl hydride in the product stream or the liquid flow rate of the product stream are unknown or change with time.
To date, the analytical methods used to determine the degree of conversion of cobalt carbonyl hydride to cobalt salts have included wet-chemistry techniques and/or well known instrumental techniques including atomic adsorption and emission spectroscopy, ultraviolet spectroscopy, and infrared spectroscopy. Each of these methods suffers from drawbacks which make their utilization undesirable. For example, the wet chemistry analytical methods are cumbersome, time-consuming, and ill suited for use as a means of feedback in a continuous air decobalting unit. Measurements based on atomic absorption and emission spectroscopy determine only the total cobalt concentration, and, thus, do not provide any information on the degree of conversion of cobalt carbonyl hydride to cobalt salts. Measurements using ultraviolet or infrared spectroscopy require either cumbersome sample preparation which would be ill suited for a continuous system or expensive high-pressure windows, since the stream is under pressure.
Thus, there is a need for a method of determining the amount of acidic metal carbonyl hydride in the product stream of a catalytic process which can be used as a means of feedback in a continuous catalyst removing unit. In particular, there is a need for a method of measuring the degree of conversion of cobalt carbonyl hydride to cobalt salts in the product stream of the oxo process which can be used as a means of feedback in a continuous air decobalting unit.